(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery and a method of preparing the same in which the positive active material exhibits improved structural and thermal stability.
(b) Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on rechargeable lithium batteries are actively being pursued in accordance with the increased need for a battery having a high energy density for use as a power source in these portable electronic instruments.
Rechargeable lithium batteries use material into or from which lithium ions are reversibly intercalated or deintercalated as negative and positive active materials. For an electrolyte an organic solvent or polymer is used. Rechargeable lithium batteries produce electrical energy from changes of chemical potentials of the active material during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early period of development. However, the lithium negative electrode degrades due to a chemical reaction with the electrolyte. Lithium dissolved in an electrolyte as lithium ions upon discharging is deposited as lithium metal on the negative electrode upon charging. When charge-discharge cycles are repeated, lithium is deposited in the form of dendrites which are more reactive toward the electrolyte due to an enhanced surface area, and they may also induce a short circuit between the negative and positive active materials and even cause an explosion of the battery in the worse case. Such problems have been addressed by replacing lithium metal with carbon-based materials such as an amorphous or crystalline carbon. The carbon-based materials reversibly accept and donate significant amounts of lithium without affecting their mechanical and electrical properties, and the chemical potential of lithiated carbon-based material is almost identical to that of lithium metal.
For the positive active material in the rechargeable lithium battery, a metal chalcogenide compound into or from which lithium ions are intercalated or deintercalated is used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1xe2x88x92xCoxO2 (0 less than x less than 1), or LiMnO2. Mn-based active materials such as LiMn2O4 or LiMnO2 are the easiest to prepare and they are less expensive and much more environmentally friendly than the other materials, but they have significantly smaller capacities than the other materials. LiCoO2 exhibits good electrical conductivity of 10xe2x88x922 to 1 S/cm at ambient temperatures, as well as high cell voltage and good electrochemical properties. Therefore, it is widely used in commercially available rechargeable lithium batteries, although the cobalt-based active material is relatively more expensive than the other materials. LiNiO2 has an advantage of having the highest specific capacity of all, but it is relatively more difficult to synthesize in the desired quality level and is the least stable of all.
These composite metal oxides are manufactured by a solid-phase method. The solid-phase method involves mixing solid raw material powders and sintering the mixture. For example, Japanese Patent publication No. Hei 8-153513 discloses a method in which Ni(OH)2 is mixed with Co(OH)2 or mixed hydroxides of Ni and Co are heat-treated, ground, and then sieved to produce LiNi1xe2x88x92xCoxO2 (0 less than x less than 1). In another method, a reactant mixture of LiOH, Ni oxide and Co oxide is initially heated at 400 to 580xc2x0 C., and then the heated reactant is heated again at 600 to 780xc2x0 C. to produce a crystalline active material.
Another way to produce such composite metal oxides is disclosed in Japanese Patent Laid open Hei. 9-55210 (Sony). In this method, a Ni-based material is coated with a metal alkoxide to prepare a positive active material. The metal alkoxide includes Co, Al or Mn. However, the metal coatings in the Ni-based material did not show improvements in the cell performance, e.g. capacity and voltage.
In addition, U.S. Pat. No. 5,705,291 (Bell Communications Research, Inc.) discloses that LiMn2O4 is mixed with aluminum oxide and briefly heated to obtain an active material with improved behavior. However, this material did not provide with a sufficient improvement in cell performances to satisfy commercial needs.
Therefore, there is sufficient need to develop positive active materials with improved structural and thermal stability, capacity, and cycle life.
It is an object of the present invention to provide active material for a rechargeable lithium battery with improved structural and thermal stability, capacity and cycle life.
It is another object of the present invention to provide a method of preparing the positive active material for a rechargeable lithium battery.
To achieve the above objects, the present invention provides a positive active material for a rechargeable lithium battery including a core comprising at least one compound represented by Formula 1 and a protective active metal oxide shell formed on the core, the active metal oxide being capable of stabilizing a structure of the active material:
Formula 1
LiA1xe2x88x92xxe2x88x92yBxCyO2,
where 0xe2x89xa6xxe2x89xa60.3, and 0xe2x89xa6yxe2x89xa60.01;
A is an element selected from the group consisting of Co and Mn;
B is an element selected from the group consisting of Ni, Co, Mn, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Cu and Al; and
C is an element selected from the group consisting of Ni, Co, Mn, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Cu and Al.
The positive active material is obtained from a process of preparing crystalline powder or semi-crystalline powder represented by Formula 1, coating the crystalline or semi-crystalline powder with a metal alkoxide suspension, and heat-treating the coated powder.